1. Field of the Invention
This invention relates to a signal reproducing apparatus and a signal reproducing method for reading out signals recorded along with clock signal components on a recording medium. The present invention is applicable to such signal reproducing apparatus in which fluctuation may be incurred in the relative velocity between the signal recording medium, such as a magnetic tape, and signal readout means, such as a rotary head.
2. Description of the Related Art
There have hitherto been known a variety of signal reproducing apparatus, such as those employing magnetic tapes, magnetic discs or optical discs, as recording media. In a majority of cases, the relative velocity between the signal readout means, such as the magnetic head or the optical pickup head, and the recording medium, is servo-controlled to a preset format velocity. The frequency of the clock signal components of the signals reproduced from signal readout means is also servo-controlled to a constant value.
There is a system consisting in storing signals read from a recording medium in a buffer memory for subsequent reading at a constant rate for reproduction. With such system, the signals may be effectively reproduced even though the relative velocity between the recording medium and signal readout means is fluctuated to a certain extent.
In the case of a small-sized rotary head type digital tape recorder (DAT), as electronic equipment, it may be contemplated to run the capstan and a rotary head in rotation by a sole motor. If the tape running speed is varied with such apparatus, the rotational velocity of the rotary head and hence the relative velocity between the tape and the head is also changed.
If the relative velocity between the recording medium, such as the tape, and the signal readout means, such as the rotary head, is changed, the frequency of the clock components of the signals obtained from the signal readout means is also changed. Such fluctuations in frequency tend to produce disengagement from a locking range of a phase locked loop circuit (PLL circuit) employed in the clock reproducing circuit (clock signal extracting tuning circuit) to render it impossible to carry out effective clock reproduction.
In the PLL circuit in general, a capturing range which is centered about the clock frequency at the time of standard reproduction and which has a frequency of transition to a locked state as a boundary frequency, and a locking range which is outside the capturing range and which has a frequency of disengagement from the locking state as a boundary frequency, are set. If the relative velocity of the rotary head with the tape is changed significantly so that the clock frequency is outside the locking range, the PLL is disengaged from the locked state so that the normal operation cannot be achieved. If the fluctuations of the clock components in the playback signals cannot be absorbed sufficiently by the PLL circuit, adverse effects such as increased signal error rate is incurred.
If, in a signal reproducing system in which tape running and head rotation are realized by separate motors, signals are read while the tape is fed rapidly or rewound to retrieve a desired tape position, the conventional practice has been to control the rotational velocity of the rotary head for compensating fluctuations in the playback clock frequency. Since a certain setting time is required for correcting the rotational velocity of the rotary head at the time of operational transition to the rapid feed or rewind, quick transition to the new operating state cannot be achieved.
In the case of a servo operation of maintaining a constant relative velocity between the signal recording medium and the signal readout head, control operations of acceleration and deceleration are repeatedly performed with increase load on the servo or circuit system with concomitant power consumption.